Using unpkg.com as an example they redirect from the root URL of a package to the latest version. For instance https://unpkg.com/mithril will redirect to https://unpkg.com/mithril@1.1.4/mithril.js

For example if adding a URL on https://flems.io it will store the xhr.responseURL to take advantage of this to pin the version. I would guess guess Scrimba does something similar?